Take a close look at the latest image from the Hubble Space Telescope. It shows a huge elliptical galaxy called NGC 474, which is about 100 million light-years away.
At about two and a half times larger than our Milky Way Galaxy, it really is a behemoth. Note the strange structure – mostly characterless and almost round, but with layered shells wrapped around the central core.
Astronomers want to know what caused these shells. The answer could lie in what this galaxy represents: a vision of the future Milky Way and the Andromeda Galaxy.
The Fate of the Milky Way: When Galaxies Collide!
Galaxies change over long timescales. More than 13 billion years ago, the first were tiny bits of matter. They merged to form increasingly larger structures. That process of amalgamation and cannibalization continues to this day.
It affects the ‘appearance’ of a galaxy and adds variety to its stellar populations. Our own Milky Way is part of that process. It is currently cannibalizing the Sagittarius Dwarf System.
It has also fused with or swallowed between 5 and 11 smaller ones during its lifetime.
Astronomers already know that the Milky Way will continue to be part of the process of fusion of galaxies.
In 4.5 to 5 billion years, it will begin to merge with the nearby Andromeda Galaxy (M31). Of course, M31 will have gotten a lot closer to us in the intervening time.
As an added bonus, the Triangulum Galaxy (M33) can also participate in this galactic dance.
For those of you keeping track of these things, this will happen when the sun runs out of hydrogen at its core and starts evolving into a red giant. So it will be an interesting time. Mark your calendars.
NGC 474 predicts the future of the Milky Way
Full Image Credit: NASA, ESA and D. Carter/Liverpool John Moores University; Image Processing: G. Kober/NASA Goddard/Catholic University of America
NGC 474 is very similar to what astronomers think the Milky Way and Andromeda will look like after their merger. It won’t be two pretty spirals anymore. Instead, their gravitational interaction will produce an almost featureless elliptical galaxy.
How will that happen? As the two galaxies approach each other, the strong gravitational pull of each will distort their shape. Giant streamers of gas and dust will be pulled from every galaxy. There may even be central sheaths of material, just like in NGC 474.
In addition to all that activity, there’s another hallmark of a fusion: starburst nodes. They are sites of star formation that take place in the wake of a merger.
The activity pushes gas and dust clouds together, eventually forming hordes of hot, young stars. That will happen as long as enough material is available for the starbirth nurseries.
Eventually, the eruption of star birth will slow down and stop. The resulting new galaxy will take on a rather dull-looking elliptical shape.
That, in a nutshell, is what happened to NGC 474. And it is the fate of Milkdromeda: a (probably) featureless elliptical trainer that was once two beautiful spiral galaxies.
Explanation of those shells in NGC 474
In the case of NGC 474, astronomers have a few theories as to why it has these strange shells. One idea is that it interacted with another galaxy billions of years ago. That created the shells in a process akin to throwing a rock into a pond and watching the ripples clear from it.
NGC 474 isn’t the only one with collision-induced grenades. About 10 percent of all ellipticals have these features. That could be a clue to their formation and fusion history that astronomers will investigate.
There’s something else interesting about these shelled galaxies. While most ellipticals are in clusters, these eccentrics take up relatively empty chunks of space.
They may have cannibalized nearby galaxies, clearing their neighborhoods of galactic competition.
Other theories about NGC 474
Full Image Credit: DES/DOE/Fermilab/NCSA & CTIO/NOIRLab/NSF/AURA Credits: Image Processing: DES, Jen Miller (Gemini Observatory/NSF’s NOIRLab), Travis Rector (University of Alaska Anchorage), Mahdi Zamani & Davide de Martin
It is also possible that NGC 474 is taking gas away from a nearby one called NGC 470.
Another idea is that the shells could have been caused by a collision with a very gas-rich galaxy. Not only did they meet once, but they had a second clash that led to their final merger.
The shells are evidence of that long fused galaxy. Hubble’s gaze gives a more detailed view of that central area and those mysterious shells.
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